Unravelling rootstock×scion interactions to improve food security

Abstract

While much recent science has focused on understanding and exploiting root traits as new opportunities for crop improvement, the use of rootstocks has enhanced productivity of woody perennial crops for centuries. Grafting of vegetable crops has developed very quickly in the last 50 years, mainly to induce shoot vigour and to overcome soil-borne diseases in solanaceous and cucurbitaceous crops. In most cases, such progress has largely been due to empirical interactions between farmers, gardeners, and botanists, with limited insights into the underlying physiological mechanisms. Only during the last 20 years has science realized the potential of this old activity and studied the physiological and molecular mechanisms involved in rootstock×scion interactions, thereby not only explaining old phenomena but also developing new tools for crop improvement. Rootstocks can contribute to food security by: (i) increasing the yield potential of elite varieties; (ii) closing the yield gap under suboptimal growing conditions; (iii) decreasing the amount of chemical (pesticides and fertilizers) contaminants in the soil; (iv) increasing the efficiency of use of natural (water and soil) resources; (v) generating new useful genotypic variability (via epigenetics); and (vi) creating new products with improved quality. The potential of grafting is as broad as the genetic variability able to cross a potential incompatibility barrier between the rootstock and the scion. Therefore, understanding the mechanisms underlying the phenotypic variability resulting from rootstock×scion×environment interactions will certainly contribute to developing and exploiting rootstocks for food security.

Keywords: Epigenetics; graft-mobile signals; microRNAs; phytohormones; transgrafting; vigour..

Fig. 1.

Phenotypic variability in grafted plants. The phenotype is more complex than in arable crops since it combines two different genotypes, causing R×S×E interactions that are driven by communication between the rootstock and the scion. The effect of the graft union itself partially mediates the R×S interaction. These interactions determine the positive or negative influence of the rootstocks on plant performance and fruit quality of the scion. PVPR, plant-growth-promoting rhizobacteria.

Fig. 2.

(A–F) Shoot fresh weight frequency distribution of tomato plants (Solanum lycopersicum cv. Boludo F1) grafted onto a population of recombinant inbred lines (RILs) from a cross between Solanum lycopersicum×Solanum pimpinellifolium (Villalta et al., 2008; Asins et al., 2010) phenotyped within the framework of the EU Project ROOTOPOWER (#289365) under control (A) and different abiotic stress conditions: low (1mM) potassium experiment (B), performed at the CEBAS-CSIC by Dr Francisco Pérez-Alfocea and his research team; low (0.1mM) phosphorus experiment (C), performed at the EEZ-CSIC by Dr Juan Manuel Ruiz-Lozano and his research team; low (2mM) nitrogen experiment (D), performed at the Université catholique de Louvain by Dr. Xavier Draye and his research team; drought stress experiment (E), performed at the Lancaster University by Dr Ian C. Dodd and his research team; and high mechanical impedance (F), performed at the Cranfield University by Dr Andrew Thompson and his research team. (G, H) Leaf fresh weight (G) and fruit yield (H) frequency distribution of a population of tomato plants (Solanum lycopersicum cv. Boludo F1) grafted onto a RILs population from a cross between Solanum lycopersicum×Solanum cheesmaniae grown under moderate salinity (75mM NaCl, replotted from Albacete et al., 2009). ∆ indicates fold change in the measured parameter between the highest and lowest vigour-inducing lines.

Fig. 3.

Relative change in stomatal aperture ratio (SAR), stomatal conductance (gs) and whole-plant transpiration (ET) of self- and reciprocally grafted wild-type (WT) and abscisic acid (ABA)-deficient mutants (aba), where 100% represents the value for self-grafted (aba/aba=scion/rootstock) ABA-deficient mutants. Specific graft combinations were WT Ailsa Craig (Jones et al., 1987; Dodd et al., 2009) and Rheinlands Ruhm (Holbrook et al. 2002) tomatoes, and the sitiens (Jones et al., 1987; Holbrook et al., 2002) and flacca (Dodd et al., 2009) mutants grown in well-watered (WW) and drying (Dry) soil and at high (92%) and low (75%) relative humidity (RH), and WT Columbia A. thaliana and aba2 mutant grown under control (MS) and osmotic (Osm) stress where Ψ=–1.0MPa (Christmann et al. 2007).

Fig. 4.

Pictures showing a mature leaf (A, B), and the 7th (C, D) and 2nd (E, F) fruit trusses of two ABA (sp12 and sp5, left panel) and CK (IPT_F and IPT_G, right panel) overproducing tomato lines and their respective WTs (Ailsa Craig: AC and UC-82B) used as rootstocks of a commercial tomato variety (Sugar Drop: SD) grown under a low salinization regime (3 dS m^–1, equivalent to 30mM NaCl) in a commercial greenhouse for 100 d.

Fig. 5.

Total fresh weight (FW; A) and appearance (B) of reciprocally grafted tomato plants, between the commercial F1 variety TT115 and the dwarf cultivar Micro-Tom (MT).

Fig. 6.

Schematic representation of rootstock-induced virus resistance in plants. A silenced rootstock expressing a small RNA homologous to a region of the viral genome produces a double strain dsRNA that moves to the scion and initiates the siRNA silencing signal that interferes with replication of the viral genetic machinery, inducing systemic silencing and virus resistance.

Fig. 7.

Schematic figure explaining the methods for obtaining graft chimaeras (upper panel) and graft hybrids (lower panel) (adapted from Liu, 2006. Historical and modern genetics of plant graft hybridization. Advances in Genetics 56, 101–129. Copyright 2006, with permission from Elsevier). A branch of the scion is grafted onto the rootstock (A), the scion is cut at the graft union (B), to allow the growth of a sectorial graft chimaera from the junction (C). The ‘mentor graft’ method consists of grafting a young seedling (cotyledonary stage) onto an old rootstock (D) and eliminating all the leaves except the two to three upper ones from the scion during the growing period (E).